Synthesis of Hyperbranched Poly(Tert-Butyl Acrylate) by Self-Condensing Atom Transfer Radical Polymerization of a Macroinimer

846 Macromol. Rapid Commun. 21, 846–852 (2000)

Communication: Using 2-hydroxyethyl a-bromoisobutyrate as initiator, atom transfer radical polymerization (ATRP) of tert-butyl acrylate leads to poly(tert-butyl acrylate) (PtBA) with a hydroxyl group at one and a bromine atom at the other end. Esterification of the hydroxyl group of these heterotelechelic polymers with acryloyl chloride — yields PtBA (Mn = 3060) with a polymerizable double bond at one end and a bromine atom at the other end which can act as an initiator in ATRP (“macroinimer”). Self-condensing ATRP of such a macroinimer leads to hyperbranched or highly branched PtBA. The polymer was characterized by GPC viscosity measurements. Even — — at M = 78800, a rather low polydispersity index of M / — w w Mn = 2.6 was obtained. A significantly lower value for the Mark-Houwink exponent (a = 0.47 compared to a = 0.80 for linear PtBA) indicates the compact nature of the Mark-Houwink plots for linear PtBA (9) and branched PtBA F branched macromolecules. ( ) and the contraction factor of intrinsic viscosity for branched PtBA (H). Column set II

Synthesis of hyperbranched poly(tert-butyl acrylate) by self-condensing atom transfer radical polymerization of a macroinimer

Guanglou Cheng1, Peter F. W. Simon2, Markus Hartenstein1, Axel H. E. Mu¨ller* 1 1 Makromolekulare Chemie II and Bayreuther Institut fu¨r Makromoleku¨lforschung, Universita¨t Bayreuth, D-95440 Bayreuth, Germany [email protected] 2 Institut fu¨r Physikalische Chemie, Universita¨t Mainz, D-55099 Mainz, Germany (Received: November 25, 1999; revised: March 23, 2000)

Introduction and even ring-opening polymerization11). The molecular In the last decade, dendrimers have been extensively parameters of the polymers obtained by SCVP have been studied as materials with novel physical properties1, 2). studied theoretically12, 13), showing that these hyper- These polymers have a very compact structure and can be branched polymers have a high degree of branching, DB L highly functionalized. However, commercialization of 0.5, and exhibit very broad molecular weight distributions dendrimers is prevented by the requirements of multi-step (MWD). The polydispersity index (PDI) is expected to be

reactions and intermediate purification. Less regular equal to the number-average degree of polymerization, Pn, hyperbranched polymers obtained by polycondensation partially due to the inevitable presence of oligomers. Slow are more easily available, but their preparation was addition of the inimers to a multifunctional initiator will 3–6) 14, 15) restricted to the polycondensation of ABn monomers . lead to higher DB and rather narrow MWD . It has been Recently, Fre´chet et. al.7) reported a new synthetic also shown theoretically16) and experimentally8, 16) that the method, self-condensation vinyl polymerization (SCVP), self-condensing vinyl copolymerization (SCVCP) of AB* to prepare hyperbranched vinyl polymers. Initiator-mono- monomers with conventional monomers, M, leads to highly mers (“inimers”) have the general structure AB*, where A branched polymers, allowing for control of MWD and stands for a double bond and B* for an initiating group. This degree of branching. This reaction gives way to the incor- general approach has been applied to various types of living poration of functional monomers into highly branched polymerization, i.e. cationic7), radical8, 9), group transfer10), polymers.

Macromol. Rapid Commun. 21, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1336/2000/1208–0846$17.50+.50/0 Synthesis of hyperbranched poly(tert-butyl acrylate) ... 847

Structures similar to those obtained in SCVCP should line. 2-Hydroxyethyl-a-bromoisobutyrate (HEBIB) was pre- 25) 1 d be obtained by using macromonomeric initiators pared according to the literature . H NMR (CDCl3): = 1 1 1 1 1 (“macroinimers”). The structure of a macroinimer, 4.31 ( CH2 OCO), 3.85 ( CH2 OH), 1.97–1.92 ( CH3, 1OH) ppm. A(m)nB*, differs from that of an AB* inimer only by the presence of linear monomer units, (m), acting as a spacer between initiator and monomer function. Preparation of HO-PtBA-Br by bulk ATRP of tBA The molecular parameters of polymers made from All operations were carried out under a nitrogen atmosphere. SCVP of a macroinimer have been calculated by Simon CuBr, PMDETA and tBA (molar ratio = 0.5/0.5/27) were 17) and Mu¨ller . Assuming the macroinimer to be monodis- added to a flask and stirred until the system turned homoge- perse and neglecting excluded-volume effects, the PDI of — neously green. Then the initiator HEBIB ([HEBIB]0/[tBA]0 a hyperbranched polymer with a given Mn was calculated = 1/27) was added dropwise. As soon as the initiator was to be (c + 1) times lower than that from an AB* inimer: added, the system turned homogeneously blue, indicating the 8 PDI = Pn/(c + 1), where c is the number-average degree start of the polymerization. The mixture was kept at 25 C of polymerization of the macroinimer. Since the linear for 140 min. After the polymerization, the catalyst was (m)-units reduce the number of branch points by the fac- removed by an adsorption filtration through a neutral alu- tor (c + 1), the expected DB should be correspondingly mina column, and the resulting polymer (HO-PtBA-Br) was — precipitated in a mixture of CH3OH/H2O (3/1 v/v). Yield: lower than that from an AB* inimer for a given M . How- — — — — n 62.5%; GPC: M = 2420, M /M = 1.34; 1H NMR: M = ever, macroinimer units which are incorporated into the n w n n 3060. 1H NMR (CDCl ): d = 4.17 (1CH 1OCO), 4.05 chain only by their vinyl groups are counted as branched 3 2 (1CH(CO2Bu)-Br), 3.81 (1CH21OH), 2.20 (1CH(CO2 whereas they are counted as linear in SCVP of inimers. Bu)1CH21), 2.0–1.0 (1CH3, 1CH21CH1) ppm. MALDI- As a consequence, DB at full conversion of double bonds TOF MS: m/z = 211.05 (initiator) + n N 128.17 (PtBA back- is increased by about 59% to DB L 0.74/(c + 1). This bone) + 79/81 (-Br) + 23 (Na). value is still smaller than the one obtained in SCVCP (DB X 2/c for c S 1). Preparation of the macroinimer Poly(ethylene glycol)18) and poly(tetrahydrofuran)19) macroinimers have been applied to prepare block and graft Acryloyl chloride (2.05 g, 22.6 mmol) was added dropwise branched copolymers, as well as crosslinked networks in to the mixture of HO-PtBA-Br (11.97 g, 4.53 mmol) and 8 conventional free-radical polymerization. These macroini- triethylamine (2.29 g, 22.6 mmol) in 500 ml THF at 0 C during 60 min. The mixture was stirred for 3 h at 08C followed mers behaved as macromonomers, macroinitiators and ma- by stirring at room temperature for 24 h. THF was then crocrosslinkers. Conventional free-radical homopolymeri- removed by a rotary evaporator. The produced polymer was zation of these macroinimers yielded insoluble material. purified by passing through a SiO2/Al2O3 (1/4 w/w) column In this paper, we wish to report the preparation of hyper- using THF/hexane (4/1) as mobile phase, followed by preci- — branched polyacrylates via the macroinimer technique pitation in CH3OH/H2O (3/1 v/v). Yield: 70%. GPC: Mn = — — 1 — 1 using atom transfer radical polymerization (ATRP). 2760, Mw/Mn = 1.29; H NMR: Mn = 3050. H NMR 20) ATRP allows the controlled polymerization of a wide (CDCl3): d = 6.49–5.68 (H2C2CH1CO1), 4.50–4.20 variety of vinyl monomers to obtain polymers with a vari- (1CH21OCO), 4.08 (-CH(CO2Bu)-Br), 2.20 (1CH(CO2 1 1 1 1 1 1 ety of well-defined architectures. By using AB* inimers, Bu) CH2 ), 2.0–1.0 ( CH3, CH2 CH ) ppm. MALDI- hyperbranched polystyrene8, 21) and polyacrylates22–24) have TOF MS: m/z = 265.05 (inimer) + n N 128.17 (PtBA back- been successfully synthesized via ATRP. By applying this bone) + 79/81 (-Br) + 23 (Na). approach to macroinimers, we have obtained highly branched or hyperbranched poly(tert-butyl acrylate) ATRP of the macroinimer using CuBr/PMDETA as catalyst (PtBA) without oligomers. PtBA macroinimers were also Under inert conditions, 0.0118 g (8.2 N 10–5 mol) CuBr, and obtained by ATRP using an OH-functionalized initiator. 0.0160 g (9.2 N 10–5 mol) PMDETA were added to a solution of 2.50 g (8.2 N 10–4 mol) macroinimer in 3.5 ml ethyl acetate. The reaction mixture was then immersed into an oil bath Experimental part at 408C and stirred with a magnetic bar. Samples were taken periodically and were passed through a short neutral alumina Materials column to remove the catalyst. The solvent was evaporated, tert-Butyl acrylate (tBA, BASF AG) was fractionated from the produced polymer was dissolved in benzene and freeze- CaH2 over a 1 m column filled with Sulzer packing at dried. 45 mbar, stirred over CaH2, degassed and distilled in high vacuum. CuBr (95%, Aldrich) was purified by stirring over- night in acetic acid. After filtration it was washed with etha- Analysis nol, ether, and then dried. N,N,N9,N99,N99-pentamethyldiethyl- GPC was performed using THF as eluent at a flow rate of enetriamine (PMDETA, Aldrich) and ethyl acetate (EAc, 1.0 ml/min at room temperature. Column set I: 5 l PSS SDV 99%, Aldrich) was degassed and distilled over a vacuum gel, 100 A˚ and linear: 102 –105 A˚ , 60 cm each; detectors: 848 G. Cheng, P. F. W. Simon, M. Hartenstein, A. H. E. Mu¨ller

26Jasco Uvidec 100 III with variable wavelength, Bischoff peak (a) from the overlapping peaks of (b) and (e9), the RI detector 8110. PtBA standards (PSS, Mainz) were used relative peak area is obtained as 0.52, which equals to for the calibration of column set I. Column set II: 5 l PSS half of peak (a). This indicates that every polymer chain 3 ˚ 5 ˚ 6 ˚ SDV gel, 10 A, 10 A and 10 A, 30 cm each; detectors: carries an x-bromine chain end. The same sample was Shodex RI-71 refractive index detector, Applied Biosystems then further analyzed by MALDI-TOF MS, as shown in 1000S UV diode-array detector, and Viscotek viscosity Fig. 2. Interestingly, four series are present in the spec- detector H 502B. Absolute molecular weights of branched trum. The signals of every series are separated by polymers were determined by universal calibration26) using the viscosity module of the PSS-WinGPC scientific V 4.02 128 Da, which corresponds to the molecular weight of Software package. 1H NMR spectra were recorded with a tBA unit. The most abundant series (a) obviously corre- a x Bruker AC-200 spectrometer at room temperature in CDCl3. sponds to PtBA with an -hydroxyl group and an -bro- MALDI-TOF MS spectra were recorded on a Micromass mine group. The first minor series (c) is assigned to the TofSpecE mass spectrometer which is equipped with a nitro- structure devoid of terminal bromine. The second minor gen laser source delivering 4 ns pulses at k = 337 nm. Dihy- series (b) might be attributed to a methoxy end group, droxybenzoic acid was used as matrix and sodium trifluoro- and the last minor series (d) to a phenoxy group. These acetate as cationization agent. three peaks have also been observed in the tBA/CuBr/ PMDETA/2-ethyl-a-bromopropionate system at 608C27) which might result from side reactions in ATRP and/or Results and discussion reaction with the matrix during ionization in the MALDI- TOF process. Since we do not see unexpected NMR sig- Characterization of HO-PtBA-Br and macroinimer nals we prefer the latter explanation. Fig. 1 presents the 1H NMR spectrum of PtBA obtained Fig. 3 presents the 1H NMR spectrum of the product from bulk ATRP of nBA using 2-hydroxyethyl a-bromo- after esterification of HO-PtBA-Br with acryloyl chlor- isobutyrate (HEBIB) as initiator and CuBr/PMDETA as ide. Compared to Fig. 1, resonances due to the methylene catalyst at 258C. The large peak (e) at 2.20 ppm is protons adjacent to the hydroxyl end from the initiator assigned to the methine protons existing in the repeat unit moiety at 3.81 ppm have disappeared completely, and of the main chain while the small peak (a) at 3.81 ppm is three new peaks (a9 and a99) appear between 6.49 and attributed to the methylene protons from the initiator moi- 5.68 ppm which are attributed to the vinyl protons. This ety at the a-end of the PtBA chain. From the peak inten- indicates a successful esterification of HO-PtBA-Br by — sity ratio of (e) to (a) we calculated Mn = 3060. This value acryloyl chloride. Furthermore after esterification, the — — — is 26% higher than the GPC value (Mn = 2420, Mw/Mn = methine proton adjacent to the bromine end appears at 1.34), probably due to the lack of suitable oligomer stan- 4.08 ppm, and the ratio of this peak to peak (e) which has dards in the low molecular weight range. No peaks been assigned to the methine proton in the polymer back- between 6.5 and 5.5 ppm were observed indicating the bone leads to a molecular weight of 3 050. This agrees absence of double bonds. Furthermore, after subtracting well with that from Fig. 1.

Fig. 1. 1H NMR spectrum of HO-PtBA-Br Synthesis of hyperbranched poly(tert-butyl acrylate) ... 849

Fig. 2. MALDI-TOF mass spectrum of HO-PtBA-Br

Fig. 3. 1H NMR spectrum of PtBA macroinimer

The success of esterification was further confirmed by mine group capable of initiating ATRP, i.e. a macroini- the MALDI-TOF spectrum, as seen in Fig. 4. Compared mer with a number-average degree of polymerization of c to Fig. 2, all four signals move to higher m/z values. The = 22 (after subtraction of end groups). mass increment (54.0 Da) exactly equals the difference between 1OH (m/z = 17) and 1OCOCH2CH2 (m/z = 71) groups. In conclusion, heterotelechelic PtBA was successfully Self-condensing ATRP of the macroinimer synthesized. The esterification of such a polymer yielded The macroinimer formed was submitted to self-conden- a PtBA with polymerizable a-acryloyl group and x-bro- sing ATRP by adding CuBr/PMDETA in ethyl acetate at 850 G. Cheng, P. F. W. Simon, M. Hartenstein, A. H. E. Mu¨ller

Fig. 4. MALDI-TOF mass spectrum of PtBA macroinimer

Fig. 6. Conversion of the macroinimer in a linear (NNNFNNN) and Fig. 5. Eluograms of PtBA formed in self-condensing ATRP first-order (-f-) plot of PtBA macroinimer. Column set I. Residual macroinimer is shown as dotted lines times, the conversion of macroinimer can be calculated. 408C. Samples were taken from the reaction and were Fig. 6 shows both the linear and first-order time-conver- analyzed by SEC to monitor the polymerization process. sion plots. It can be seen that the concentration of the pro- As can be seen from Fig. 5, the molecular weight of the pagating active species is constant during the polymeriza- produced polymer increases steadily with reaction time. tion and side reactions are insignificant. Although conver- — After 24 h, GPC indicates complete conversion of the sion of the macroinimer is completed after 24 h, Mn of the macroinimer. The final polymer obtained after 48 h has formed polymer still increases indicating further coupling an apparent number-average molecular weight of 20940 reactions between different individual branched macro- and a polydispersity index of 2.33, calibrated against lin- molecules (Fig. 7). ear PtBA standards. In contrast to the SCVP of AB* In order to obtain true molecular weights, a viscosity inimers7), distinct peaks of dimer, trimer, tetramer, etc. detector and universal calibration were used26). Fig. 7 were not observed even in the early stages of the poly- demonstrates the relationship between reaction time and — merization. This may be due to the fact that the macroini- true and apparent Mn of the branched polymer formed mer already has a polydispersity index of 1.29. during the polymerization. The true molecular weights By subtracting the normalized GPC peak of the macro- are considerably higher than the apparent ones obtained inimer from that of the polymers at different reaction by GPC against linear PtBA standards indicating a lower Synthesis of hyperbranched poly(tert-butyl acrylate) ... 851

polymer obtained after 48 h. At highest molecular weight, the intrinsic viscosity of the branched polymers is less than 40% of that of the linear one. A lower value of the Mark-Houwink exponent for the branched PtBA was obtained (a = 0.47 compared to a = 0.80 for linear PtBA). This value is higher than that from the self-condensing GTP of 2-(2-methyl-1-triethylsiloxy-1-propenyloxy)ethyl methacrylate (MTSHEMA) (a = 0.32)10) indicating the expected lower DB of the branched PtBA in the present system. Furthermore, this value can be correlated to that from the SCVCP (via GTP) of MMA with MTSHEMA (a = 0.4 at c = 26)28). Here, c stands for the molar ratio of

— monomer to inimer which can be regarded as hypotheti- Fig. 7. Dependence of Mn and PDI of the obtained PtBA on the cal degree of polymerization of a linear PMMA initiated reaction time in self-condensing ATRP of PtBA macroinimer — (-f- and NNN0NNN), true M and PDI from column set II obtained by by the inimer MTSHEMA. Although care should be n — universal calibration; -F- and NNN9NNN), apparent Mn and PDI from taken when comparing the Mark-Houwink exponents of column set I) PtBA and PMMA, this seems to indicate that SCVCP leads to higher DB at a comparable c value. However, this advantage is diminished by the existence of oligomers in the final polymer. Hyperbranched PtBA is easily hydrolyzed to result the corresponding hyperbranched poly(acrylic acid). Charac- terization of these polyelectrolytes is in progress.

Acknowledgement: We wish to thank Mrs. Karin Bach (group of Prof. M. Schmidt, Universita¨t Mainz) for measuring the MALDI-TOF spectra. This work was partially supported by the Deutsche Forschungsgemeinschaft within the Schwerpunktspro- gramm “Polyelektrolyte”.

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